Heretofore, in order to perform rolling and other processing ancillary thereto on steel strips (hereinafter referred to as rolling processing) in a steel strip processing line, between two adjacent steel strips continuously supplied to the steel strip processing line, an end-to-end connection is achieved by butting and welding their opposite end edges. Also, such processing and rewinding, welding, slitting and cutting (hereinafter referred to as conditioning processing) are performed on steel strips of silicon steel, and then resultant steel strips are supplied as a product to the customer. Especially, in order to obtain a product having a unit weight required by the customer, steel strips are joined together by butt welding, thereby obtaining continuous steel strips having an end-to-end connection therein and cutting continuous steel strips as the product having said unit weight and then supplying to the customer the product in the form of coils or sheets. Therefore, the product, without aftertreatment of the weld portion formed during such conditioning processing is delivered to the customer, so that it is required to obtain a weld portion of high quality. Particularly, from the standpoint of reducing power loss, strips of silicon steel are required to be rolled to a thickness of about 0.35 mm or less. Therefore, such thin steel strips are required to be welded together before they are supplied to the customer. However, as to such thin strips, it is impossible to obtain a weld portion of high quality, as above described, by edge-preparation of cutting opposite ends of adjacent strips using the prior art shearing technique and then by butt-welding the resultant end edges thereof by prior art arc welding, such as TIG or MIG.
Namely, in both the former case of rolling processing and the latter case of conditioning processing between adjacent steel strips, the trailing end of a preceding steel strip and the leading end of a succeeding steel strip are cut by using the prior art shear welder forming a continuous process, and then the cut end edges opposed each other are caused to be butt-welded by the arc welding, i.e., TIG or MIG. In other words, in a continuous processing line constructed by the prior art shear welder, steel strips are caused to run from the upstream to the downstream, and there, are provided along this flow of steel strips, a cutting unit and a welding unit of steel strips at first end of the steel strip is cut by the shearing unit existing on the upstream, and then the strip is transferred to the downstream, where the opposite cutting ends between adjacent steel strips are butted and welded by the arc welding technique, such as TIG or MIG.
However, in the case of the arc welding, i.e., TIG or MIG, a large thermal influence zone is generated and also the width of the weld bead tends to be large. The arc welding technique therefore is unsuitable for butt-welding processes during conditioning processing of thin steel strips such as silicon steel strips. That is, when the MIG or TIG arc welding is used during the conditioning processing, thin steel strips with a weld portion of good quality cannot be obtained. Therefore, the joined steel strip is wound into the form of a coil without taking off the weld portion including the steel strip, flaws are generated in the steel strip, thus greatly deteriorating the quality of the product. For this reason, owing to avoid troubles with customer, it has been in practice to remove the weld portion of the steel strip, when the steel strip is wound as a coil.
Recently, high performance laser welding processes and laser welders have been developed, and a high quality weld portion can be obtained by using the laser welding process. When the high performance laser welding is used for butt welding during conditioning processing of a thin silicon steel strip, a continuous steel strip having a weld portion of high quality can be obtained, so as to be wound in the form of a coil without removing the weld portion. By so doing, no substantial flaw is generated.
Further, the laser welding process permits sufficient effects and great advantages to be obtained when it is used for butt welding during the conditioning processing of silicon steel strips. That is, the weld bead of the weld portion obtained by laser welding has a very small width and the weld portion is substantially free from thermal influence. This means that it is not necessary in a product of laser welding to remove the weld portion on the jointed steel strip, which may be punched out by the customers during a continuous stamping out of patterns on the sheet, without applying any treatment to the weld portion by themselves. This point is very advantageous for the customers. Actually, punching of silicon steel strips is usually processed in high speed continuous stamping equipment which is proved by the customer. Therefore, very long jointed steel strips are strongly desired for increasing the punch efficiency on the side of the customer.
However, the advantage of the effective laser welding as noted above cannot be obtained simply by a substitution of a laser welding torch in place of the TIG or MIG welding torch of the prior art shear welder. In other words, unless edge preparation by cutting opposed ends of steel strips and butting of cut end edges opposing each other is done with high accuracy, the desired effects of the laser welding cannot be obtained, so that a high quality weld portion cannot be obtained. For the sake of reference, a high quality weld portion which meets the customer's demands is defined as follows:
(a) that weld portion is homogenous and conforms to a predetermined reference,
(b) that mechanical characteristics of weld portion conforms to those decided by predetermined reference, and
(c) that the weld portion is free from bends or failure of alignment of end edges of steel strips (i.e., failure of alignment means that a line, which is formed by butted and edges of steel strips, is not conformity with a straight line perpendicular to the center line of the shear welder).
For reducing ore loss, there is a trend for reducing the thickness of silicon steel strips, particularly directional steel strips having crystalline anisotropy, from about 0.35 to 0.30 mm to about 0.35 to 0.15 mm or below. The smaller the thickness, the more difficult it is to cut steel strip ends with high accuracy and butt the cut end surfaces with high accuracy and without formation of any gap.
Further, where rolling and heat-treatment are applied to cold-rolled thin steel strips, which are rewound from a coiled form, before being supplied as a product, steel strips to be rewound are joined together by welding, in order to perform a continuous rolling operation. In this case, in the past, by using the TIG, MIG and like welding processes, the rewound cold-rolled steel strips were joined to each other, but recently it has been tried to use the laser welding process which features a high energy density. The reason for this is the same as for silicon steel strips. That is, with laser welding it is possible to provide as small laser beam diameter as up to about 0.l mm. It is thus possible to increase the energy density and reduce thermal influence, so that weld beads having a flat surface can be readily obtained.
This means that a laser welding permits welding of cold-rolled thin strips having a thickness of 0.05-1.0 mm, such as stainless steel strips and silicon steel strips. Such thin steel strips are incapable of being welded by the prior art TIG or MIG welding processes. Even in this case, however, the use of the laser welding requires highly accurate cutting and butting prior to butt welding. Particularly, it is important to cut ends of steel strips accurately along a straight line perpendicular to the center line of the steel strips in conformity with one of continuous processing line and effect the butting of the cut ends such as not to produce departure from the alignment of the end edges of the butted steel strips.
To meet these requirements, there have been developed techniques of accurately cutting ends of steel strips and causing accurate butting of the cut ends prior to butt-welding. One such technique is disclosed in Japanese Utility Model Laid-Open No. 57-102495.
Namely, in order to avoid departure from the alignment of weld line formed by butt-welding, the cut end edges of steel strips should be straight and perpendicular to the center line of the steel strips, i.e., perpendicular to the direction of travel of the continuous process line. In addition, during butting resultant cut edges of steel strips, a straight alignment should be formed between cut edges opposed each other. The Japanese Utility Model Laid-Open No. 57-102495 disclose an aligning device for securing one side edge of a steel strip such that it is parallel to the center line of the steel strip. In this aligning device, a side guide is secured to one side of a work table at a cutting or welding position of a continuous processing line, so that the side guide is parallel to the center line of travel of the continuous processing line. One side edge of the steel strip is urged against the side guide, thereby making an alignment of the center line of steel strip with the center line of travel of the continuous processing line. Therefore, in such alignment the cutting or butting of the steel strip is done prior to welding.
However, since this aligning device moves the steel strip with the press (conveyor) belt in contact with the surface of the steel strip, press flaws are liable to be formed on the surface of the steel strip, particularly the surface of a very thin steel strip, by the urging force of the press belt and movement thereof in the widthwise direction of the work table. Further, when the steel strip is very thin, it is liable to adhere to the surface with the work table on the continuous processing line such as on a shear welder. Therefore, it is difficult to move the steel strip in the widthwise direction towards the side guide for alignment by pressing against the surface of the steel strip. Therefore, even if this aligning device is used, it is difficult to obtain a weld portion free from any edge alignment departure.
Denoting the coefficient of friction between the press belt and steel strip by .mu..sub.1, the coefficient of friction between the steel strip and the work table by .mu..sub.2, the urging force of the press belt by N, and the weight of the steel strip by W, the force, with which the steel strip is pulled by the process belt, is EQU N.mu..sub.1 -(N+W).mu..sub.2 =N(.mu..sub.1 -.mu..sub.2)-W.sub.2
This relation means that in order for pulling the steel strip by the press belt it is necessary to meet a condition .mu..sub.1 greater than .mu..sub.2.
Further, since the weight W of the steel strip varies with the thickness and width of the strip, it is necessary to adjust the urging force N of the press belt. Furthermore, with a thin steel strip, an excessive pulling force will cause a strain of the strip after the side edge thereof has been brought into contact with the side guide. In such a case, the side edges of the two steel strips will not be truly straight after subsequent cutting and butting. A desired accuracy of the joint thus cannot be obtained.
In the meantime, in order to stop the steel strip as soon as one side edge thereof is brought into contact with the side guide of the work table it may be contemplated to use a high accuracy sensor, a continuous motor, etc. However, such equipment is expensive and poses many problems in practice, so that it has not yet been employed in practice.